The invention relates to methods and apparatus for monitoring characteristics of a flow stream in a pipeline, in particular to dual chamber orifice fittings. More precisely, the embodiments of the invention relate to an improved system for positioning an orifice plate within the fitting.
In pipeline operations and other industrial applications, flow meters are used to measure the volumetric flow rate of a gaseous or liquid flow stream moving through a piping section. Flow meters are available in many different forms. One common flow meter is an orifice meter, which includes an orifice fitting connected to the piping section. The orifice fitting serves to orient and support an orifice plate that extends across the piping section perpendicular to the direction of flow stream. The orifice plate is generally a thin plate that includes a circular opening, or orifice, that is typically positioned concentric with the inner surface of the piping section.
In operation, when the flow stream moving through the piping section reaches the orifice plate, the flow is forced through the orifice, thereby constricting the cross-sectional flow area of the flow. Due to the principles of continuity and conservation of energy, the velocity of the flow increases as the stream moves through the orifice. This velocity increase creates a pressure differential across the orifice plate. The measured differential pressure across the orifice plate can be used to calculate the volumetric flow rate of the flow stream moving through the piping section.
A dual chamber orifice fitting embodies a special design that enables the orifice plate to be removed from the fitting without interrupting the flow stream moving through the piping section. This specially designed fitting has been known in the art for many years. U.S. Pat. No. 1,996,192, hereby incorporated herein by reference for all purposes, was issued in 1934 and describes an early dual chamber orifice fitting. Fittings with substantially the same design are still in use in many industrial applications today. Although the design has remained substantially unchanged, operating conditions continue to expand and dual chamber fittings are now available for piping sizes up to 48-inches in diameter and for working pressures up to 10,000 psi.
A common dual chamber orifice fitting 12 is illustrated in FIG. 1. Orifice fitting 12 includes body 16 and top 18. Body 16 encloses lower chamber 20 which is in fluid communication with the interior 34 of pipeline. Top 18 encloses upper chamber 22 and is connected to body 16 by bolts 17. Aperture 30 defines an opening connecting upper chamber 22 to lower chamber 20. Valve seat 24 is connected to top 18 and provides a sealing engagement with slide valve plate 56, which is slidably actuated by rotating gear shaft 54. Lower drive 36 and upper drive 38 operate to move orifice plate carrier 32 vertically within fitting 12.
Orifice 31 is located on an orifice plate 33 supported by orifice plate carrier 32. Orifice plate carrier 32 is shown in a metering position in aligmnent with bore 34. To remove orifice plate carrier 32 from fitting 12 the following steps are used. First, gear shaft 54 is rotated to slide valve plate 56 laterally and away from valve seat 24 and open aperture 30. Once aperture 30 is opened, lower drive 36 is actuated to move orifice plate carrier 32 upwards into upper chamber 22. Once orifice plate carrier 32 is entirely within upper chamber 22, aperture 30 is closed to isolate the upper chamber from bore 34 and lower chamber 20. Any pressure within upper chamber 20 can then be relieved and orifice plate carrier 32 can be removed from fitting 12 by loosening clamping bar screws 46 and removing clamping bar 44 and sealing bar 40 from top 18.
The location of the orifice 31 within bore 34 is closely controlled because any misalignment may cause inaccuracies in measuring the flow through the pipeline. American Petroleum Institute (API) 14.3 sets forth the dimensional standards and tolerances for the position of orifice 31 in bore 34. Compliance with the requirements of API 14.3 adds complexity to the manufacture of conventional dual chamber orifice fittings. Many conventional designs rely on precisely positioned surfaces inside body 16 to guide and position orifice plate carrier 32. Often, several machining steps are required to place these selected surfaces in the proper relationships in order to guarantee compliance with API 14.3.
Thus, there remains a need in the art for dual chamber orifice fittings providing improved orifice plate positioning systems. The embodiments of the present invention are directed to plate alignment apparatus for dual chamber orifice fittings that seek to overcome these and other limitations of the prior art.